A conventional hearing aid or listening device can include both a microphone and a telecoil. The microphone picks up acoustic sound waves and converts the acoustic sound waves to an electrical signal. That signal is then processed (e.g., amplified) and sent to the speaker (or “receiver”) of the hearing aid. The speaker then converts the amplified signal to an acoustic signal that is broadcast towards the eardrum.
On the other hand, the telecoil picks up electromagnetic signals. The telecoil for a hearing aid is a small electromagnetic induction coil, such as a wire wound around a magnetic bobbin. The telecoil produces a voltage over its terminals when placed within an electromagnetic field, which is created by an alternating current of an audio signal moving through a wire. When the telecoil is placed near the wire carrying the current of the audio signal, an equivalent audio signal is induced in the telecoil. The signal in the telecoil is then amplified and sent to the speaker (or “receiver”) of the hearing aid for conversion to an acoustic signal.
The telecoil can be used in connection with a telephone. The telephone headset includes speakers with induction coils. If an individual places the hearing aid with the telecoil adjacent the telephone headset, an alternating current in the induction coils of the telephone speakers creates an electromagnetic field that induces an audio signal in the telecoil. The signal of the telecoil is amplified and sent to the speaker of the hearing aid. Thus, the individual receives the telephone conversation without any background audio noises.
Another use of the telecoil is to receive the sound that is passed into a microphone used by a speaker in a large room, such as a church or auditorium. The microphone, of course, sends the audio signal to loudspeakers which convert the audio signal to an acoustic signal. But, the audio signal may also pass through an induction loop around the room and create a magnetic induction field. The telecoil picks up the magnetic field and, thus, the person wearing the hearing aid can hear the speaker without the common background audio noises that may occur in a church or auditorium.
In addition to receiving the audio frequency magnetic signal from induction loops (referred to as low frequency signals—LF telecoils), the telecoil of a hearing aid may receive modulated/RF electromagnetic signals from a remote control device or programming equipment (referred to as high-frequency—HF telecoils). Thus, the HF signal may be used to control the operation of the hearing aid or to program the hearing aid. Because of the different demands on the properties of the telecoil for receiving LF and HF signals (e.g., response curve, Q, losses, size, wire diameter, etc.), there are typically two induction telecoils (i.e., HF and LF telecoils) presently used in hearing aids when both functions are required.
The current LF telecoils having integral amplifiers (“amplified telecoils”) use the amplifier in a feedback configuration that is provided by internal or external feedback devices, which forms a low-impedance current input for the telecoil as shown in FIG. 1. The frequency response from the source, eoc, is then a low-pass response and is mainly determined by telecoil parameters (e.g., inductance Lc, resistance Rc). The result is the desired, relatively flat frequency response from the inductive transmitting source to the output of the amplified telecoil (i.e., “telecoil transfer function”) in the operating audio frequency signal range of approximately 20 Hz to 10 kHz. Because the characteristic frequency response and gain of the telecoil transfer function depend on the same telecoil parameters (i.e., inductance Lc and resistance Rc of FIG. 1), it is difficult to set both the desired gain and the desired frequency response of the amplified telecoil system at the same time.